whole_WhelanJamesRobert2005_thesis.pdf (19.48 MB)
Wetdeck slamming of high-speed catamarans with a centre bow
thesisposted on 2023-05-27, 14:10 authored by Whelan, James Robert
The influence of a centre bow on the severe wetdeck slamming behaviour for a range of hull forms based on an INCAT fast catamaran ferry has been studied with full-scale trials, experimental drop tests and computationally. Particular attention was paid to the effect centre bow geometry has on the severity of slam events. Full-scale trial data from a 96m INCAT vessel was analysed. The influence of vessel Froude number, wave height, relative centre bow impact velocity, relative centre bow submergence and instantaneous wave height on the slamming behaviour of the vessel was identified. Severe slams were found to be caused by wetdeck slamming. The average relative motion between the centre bow and the instantaneous water surface during slamming was calculated and this was used for scaling of the experimental drop tests. Two-dimensional drop tests were used to experimentally investigate the behaviour of seven realistic bow geometries for catamarans with centre bow, and two vee wedge geometries. Drop tests were made under gravity with varied drop heights and mass into a long thin tank of calm water. Acceleration and surface pressure measurements and high-speed photography flow visualisation were used to detail model behaviour. The most significant observations were that the highest point in the arch should be located as far outboard as possible to minimise peak acceleration and that models with large centre bows experience peak acceleration at a lower relative initial water surface penetration depth than models with small centre bows. The trapped air in the top of the arch was also found to be important in attenuating peak acceleration. Two theoretical models were developed to model the drop tests; a one-dimensional added mass theory and a two-dimensional volume of fluid method. The added mass theory was based on von Karman's impact theory coupled with a potential flow model to account for the upwash of water during entry and a compressible air model to describe the effect of trapped air in the arch between the hulls. This theory was found to capture the trends displayed in the drop test experiments adequately. The volume of fluid method modelled the drop tests by solution of the Euler equations on a rectangular grid. The model was validated against the broken dam problem, the run up of a Laitone wave and the wedge entry problem.
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